The lustrous appearance of fiber yarns, such as those used in textile and carpet applications, can be characterized in terms of various optical parameters such as those related to reflection and refraction of impinging visible light arising a air-polymer and polymer-air interfaces of individual filaments comprising the yarn. These effects lead to subjective responses in observers of a finished carpet which, for example, may be expressed as brightness and contrast. Brightness and contrast are used herein as components of the luster appearance of a carpet along with bulk of the carpet yarn. Yarn luster is a complex fuction of the cross sectional shape of the filaments comprising the yarn.
Direct measurement of yarn luster is difficult to accomplish or to express in a mathematical sense. A discussion of yarn luster properties and its dependence on filament cross section may be found in U.S. Pat. No. 3,367,100 - Hughey, issued Feb. 6, 1968. Cross sectional shape of the filaments in a given yarn is, in turn, dependent upon the characteristics of the orifices in the spinneret plates used to produce the yarn. The actual shape imparted to a filament by the spinneret orifice is difficult to predict as is well known in the art and explained, for example, in U.S. Pat. No. 3,478,389 - Bradley et al., issued Nov. 18, 1967.
Fiber cross sectional geometries other than circular are widely employed to achieve both desired higher and lower levels of lustrous appearance of fibers for such applications. Other special geometries, such as symmetric and asymmetric multilobal cross sections, are used to impart desirable and esthetically pleasing appearance properties associated with the degree of fiber luster.
At the present time, the available processes for determining and rating such properties are largely subjective and the search for new fiber cross sections and the examination of their influence on the yarn produced from such fibers is therefore time consuming and expensive. Furthermore, because of such limitations, it has not been possible to control such properties as a part of manufacturing operations.
In addition, presently known techniques for determining analytically the actual cross sectional shape of fibers in yarns and for deriving and expressing such shapes in a mathematical format are subject to certain limitations. The conventional fiber cross sectional shape descriptors for non-round fibers are modification ratio ("MR"), the ratio of the diameters D of the circumscribed to inscribed circles, tip ratio ("TR"), the ratio of the diameters of the circle inscribed within an arm to the circumscribed circle, and arm angle ("AA"), the angle defined by tangents to the arm at the points of inflection. Such techniques are described in detail, for example, in U.S. Pat. Nos. 2,939,201 - Holland and 4,492,731 - Bankar et al. The use of the parameters MR, TR and AA serve well in many cases but are subject to certain limitations. In particular, they have limited value for even slightly asymmetrical versions of multilobal fiber products.